If I have a Vengeful Pharaoh in the graveyard, and an opponent successfully hurts me with a True-Name Nemesis, what happens with Pharoah?
If you are a "Chosen player" as your opponent has chosen when he/she cast True-name nemesis.
Then the Nemesis can't be target of your spell and/or ability.
So , Vengeful pharaoh's ability will trigger , but when you put this ability on the stack and its target is ill-legal. Pharaoh's ability just be removed from the stack.
Please autocard , check it on [read this first] - creating a rules thread.
And use lower case c to autocard.
To answer your specific question, "What happens with Pharoah?", well the Pharoah remains in your graveyard waiting to get another unsuspecting victim (one without protection from him)
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this is different than if you had No Mercy or Dread, since those don't target they will destroy the Nemesis just fine